It is well known in the art that various relatively rigid and/or brittle interpolymers of monovinylidene aromatic monomers with ethylenically unsaturated nitrile monomers can be made more impact resistant by the inclusion of amounts of various types of elastomeric materials (rubbers) into a matrix or continuous phase of said interpolymer material. Usually, the elastomeric materials are in the form of discrete particles, such particles having amounts of the matrix interpolymer, or an inter- or homopolymer similar thereto, graft-polymerized to the particles. These types of rubber-modified, impact-resistant polymeric compositions are commonly known and referred to as graft copolymers or polyblends. Among the best known of these types of compositions are the ABS or ABS-type compositions. Compositionally, ABS or ABS-type compositions generally comprise a combination of an elastomer usually containing polymerized butadiene, with a rigid interpolymer of monovinylidene aromatic monomer with ethylenically unsaturated nitrile monomer. Structurally, ABS or ABS-type compositions usually consist of the rigid, matrix or continuous phase having dispersed therein particles of the elastomer, such particles usually having grafted thereto amounts of the rigid interpolymer or a similar inter- or homopolymer.
It is also well known in the art that the physical properties of these types of compositions are greatly affected by the relative amounts of elastomer particles having different sizes and particle structures. Larger rubber particles having diameters greater than about 0.5 micron (.mu.) contribute greatly to impact resistance but tend to reduce the gloss of articles formed or molded from compositions containing them especially in the case of the below-described mass particles. On the other hand, when smaller modifying-rubber particles are used in polymer compositions, articles formed therefrom tend to be glossier but are less impact resistant than if the same amount of rubber was used in the form of larger particles.
Furthermore, concerning the structures of the individual rubber particles (i.e., rubber particle morphology) there are well-known advantages and disadvantages accompanying the use of either of the two main rubber-particle types in rubber-modified polymer compositions. It is generally believed that grafted rubber particles containing occlusions of matrix polymer therein, one of the two main rubber-particle types, provide more impact resistance than the same amount of rubber in the form of similarly grafted, solid rubber particles, the other main particle type. Such grafted, occlusion-containing rubber particles are usually formed and grafted in a mass-type or mass-suspension-type polymerization where a previously-produced rubber is dissolved in an amount of polymerizable monomer or in a mixture or solution of polymerizable monomer(s) with optional diluents, which monomer(s) are thereafter polymerized. Occlusion-containing particles, produced in such mass, mass-solution or mass-suspension processes or variations of these processes are hereafter referred to as "mass particles". It is difficult, however, using available types of rubber and mass process equipment to produce groups of mass particles having average diameters less than 0.5.mu..
It is also well known in the art that mass particles present in rubber-modified polymeric compositions can have a very detrimental effect on the gloss of articles formed therefrom. In spite of the disadvantages of mass particles, however, they are a very desirable constituent of rubber-modified polymer compositions. One basis for their desirability is that, probably due to their occluded structure, they provide a great deal of impact resistance for the amount of rubber which is actually included. Other desirable facets of including mass particles in rubber-modified polymer compositions include the ability to utilize a wide variety of rubber compositions and types and the economy and efficiency of the mass-type processes by which they are formed.
The other main type of rubber particle morphology (i.e., the above-mentioned "solid" or non-occluded rubber particles) is usually achieved via emulsion polymerization of the rubber in an aqueous latex. After the rubber is made, monomers which are polymerizable and graftable (e.g., styrene and acrylonitrile) are usually added to the rubber-containing latex and polymerized to form the graft portion as well as amounts of matrix polymer. The non-occluded type of rubber particles, produced via emulsion polymerization process, are hereinafter referred to as "emulsion-particles". When these emulsion particles have been grafted with a different, relatively rigid polymer, but still have a high rubber concentration, at least about 30 weight percent or so, these compositions are very suitable for blending with additional amounts of the same or different rigid polymer, optionally containing additional amounts of rubber, to achieve desired rubber contents in the resultant compositions. Such blendable intermediates are often referred to as "grafted rubber concentrates" or "GRC's" and can be used to produce a wide variety of rubber-modified polymer compositions.
Under most circumstances, however, emulsion polymerization techniques are generally economically feasible for the production of rubber particles having diameters less than about 0.25.mu. or so. Such particles must usually be agglomerated or coagulated in some way before, during and/or after grafting in order to achieve rubber particles having diameters greater than about 0.5 .mu.. Agglomerating and coagulating techniques are well known in the art. See, for example, U.S. Pat. Nos. 3,551,370; 3,666,704; 3,956,218 and 3,825,621; all of which are included herein by reference. A particularly desirable technique for the controlled agglomeration of the particles of an emulsion-prepared rubber in an aqueous dispersion is taught in the pending U.S. patent application, Ser. No. 350,849, filed Feb. 2, 1982, entitled "Particle Agglomeration in Rubber Latices" by D. E. Henton and T. M. O'Brien, which application is incorporated herein by reference.
As is obvious from the above discussion, and well known in the art, emulsion polymerization techniques are well-suited for preparation of smaller rubber particles while mass-type processes or agglomeration of smaller, emulsion particles can be used to achieve large particle sizes.
As is also generally known in the art, there are other individual characteristics of rubber particles, once the desired size has been determined, which can be conveniently and separately controlled to optimize certain properties of the rubber-modified polymer compositions to which they are added. Some parameters which are subject to quite wide variation to affect the physical properties of the resultant compositions include the molecular weight of the mass rubber, the degree to which either mass or emulsion rubber is crosslinked and the amounts and types of different polymers which are grafted to the particles.
In view of these phenomena observed in the production of ABS and ABS-type compositions, a great deal of effort has gone into achieving optimized physical properties by tailoring the rubber particle distributions (i.e., the sizes and types of rubber particles and the amounts of different size and/or type rubber particles) in the ABS and ABS-type compositions. See, for example, representative U.S. Pat. Nos. 3,509,237; 3,576,910; 3,652,721; 3,663,656; 3,825,621; 3,903,199; 3,903,200; 3,928,494; 3,928,495; 3,931,356; 4,009,226; 4,009,227; 4,017,559; 4,221,883; 4,224,419; 4,233,409; 4,250,271 and 4,277,574; wherein various "bimodal" particle size distributions are disclosed. As used in the art and herein, a composition having a "bimodal" particle size distribution contains two distinct groups of rubber particles, each group having a different average particle size.
The teachings of most of these patents can be broadly characterized as teaching that gloss and impact resistance are inversely related and that a gain in one is usually achieved only with a loss in the other. Most of them teach that a substantial percentage of the rubber particles must be of the small, emulsion-produced type to yield satisfactory, glossy, impact-resistant ABS and ABS-type compositions. Most important, however, is the fact that none of them teach or suggest that small, emulsion-prepared particles can advantageously be combined with both mass-produced large particles and agglomerated, emulsion-produced large particles to produce compositions having improved impact resistance and good gloss.